1. Field of the Invention:
The present invention relates to an improved method of manufacturing poly(hexamethylene adipamide), i.e. nylon 6,6, by reacting monomethyl adipate with hexamethylenediamine. More specifically but not by way of limitation, the invention relates to a process wherein methanol is removed at an early stage prior to polycondensation thus producing poly (hexamethylene adipamide) characterized as having a relative viscosity of from about 20 to about 100 (measured at 8.4 wt % polyamide in 90/10 wt % formic acid/water @ 25.degree. C.) and further characterized as containing less than 50 moles N-methylhexamethylenediamine per million grams polyamide.
2. Description of the Related Art:
Polyamides are generally prepared from salts of a diamine and dicarboxylic acid or by reaction of a diamine and a dicarboxylic acid diester. An example of the latter process is described in Yura et al. U.S. Pat. No. 3,607,840 which discloses a process for making nylon by reacting a lower alkyl diester of adipic acid with hexamethylenediamine in the presence of 20 to 70 wt % water at a temperature of less than 100.degree. C. while removing the alkanol via distillation to form an aqueous solution of the salt hexamethylene diammonium adipate. The resulting aqueous solution is heated to a temperature sufficient to remove the water followed by polymerizing the salt to form nylon 6,6 polymer. Heintzeler et al. in GB 1,053,100 discloses the reaction of diesters of adipic acid with hexamethylene diamine in the presence of 1 to 50 wt % water. In the disclosed process, it is not necessary to work under superatmospheric or subatmospheric pressure. Takahashi et al. in Japanese application JP 57-80426 discloses a process for preparing polyamides from hexamethylenediamine and the dimethyl ester of a dicarboxylic acid by reacting at a temperature of 120 to 150.degree. C. while maintaining the water concentration at 40 to 90 wt % in a sealed system until essentially all of the starting materials have reacted, removing the methanol after essentially all of the starting materials have reacted, and then polymerizing. The initial reaction is conducted in a sealed system to avoid loss of the starting materials with escaping methanol. However, heating in a sealed system leads to reaction of methanol with an amine resulting in N-methylated amine. N-methylation interferes with polymer crystallization and N-methylated amine ends are less reactive for polyamidation resulting in reduced molecular weight.
In U.S. Pat. No. 3,454,536 the above problem associated with secondary reactions and specifically the undesirable N-methylamino by-products are acknowledged. The disclosure teaches that such by-products noticeably influence the properties of the end products if they are present in a concentration of above approximately 2 mole percent, relative to the number of carbonamide groups present in the polyamide. A process for preparing polyamide from a dimethyl ester of an aromatic dicarboxylic acid and a diamine is disclosed wherein a preliminary condensate is formed in the presence of water at 90 to 100.degree. C. with simultaneous distilling off of resultant methanol-water mixture to the point of removing substantially all of the methanol formed in the reaction. This step is followed by polycondensing the distillation residue at 250 to 290.degree. C. to form the desired polyamide.
Preparation of the monomethyl ester of adipic acid, monomethyl adipate, is described in Schneider et al. U.S. Pat. No. 4,537,987. The adipic monoesters are described therein as being useful for the preparation of sebacates by Kolbe synthesis. Monomethyl adipate can be hydrolyzed to form adipic acid for use as a starting material for polyamides, as described in Kummer et al. U.S. Pat. No. 4,931,590. The isolation of adipic acid requires additional steps of crystallization and drying, resulting in additional costs. It would therefore be desirable to react monomethyl adipate directly in a nylon polymerization process.